A digital subscriber line (DSL) system may transport high-bandwidth data, such as multimedia and video, within a communication network. DSL systems typically employ twisted-pair telephone lines to transmit and receive data. Transmission distance over the twisted-pair telephone lines is inversely proportional to data rate and typically ranges from about 1000 feet for higher data rates up to several miles for lower ones. The general term DSL is used to cover a variety of similar system implementations, which have the ability to deliver high-bandwidth data rates to dispersed locations with relatively small changes in existing communication infrastructure.
A DSL system employs a transmission unit, having both transmit and receive capability, at a central office location associated with a service provider (central office equipment) and a transmission unit at a remote end location associated with a service subscriber (customer premises equipment). A VDSL1 system may define several system bandwidth options based on the inverse discrete Fourier transform (IDFT) size (parameter n corresponding to the number of subchannels employed) and the subchannel spacing (4312.5 Hz or 8625 Hz). Currently compliant systems employ upstream and downstream transmission bands that span 2.208 MHz, 4.416 MHz, 8.832 MHz or 12 MHz. A similar situation (i.e., various system bandwidth options) is also likely for a VDSL2 system.
Crosstalk is a major impairment in DSL telecommunication networks, since it degrades both upstream and downstream data communications thereby lowering effective data rates needed to provide reliable data communication. Crosstalk occurs between different DSL twisted-pair transmission lines when the signal on one twisted-pair cross-couples into another twisted-pair due to their close proximity. Crosstalk may be classified as near-end crosstalk (NEXT) or far-end crosstalk (FEXT). NEXT occurs between signals originating from multiple transmission units at the same end of a DSL pair. Alternatively, FEXT occurs between signals originating from multiple transmission units at the opposite end of a DSL pair.
Discrete multitone modulation (DMT), which is a multicarrier modulation technique using DFT to create and demodulate each individual carrier, is employed for data transport in most DSL systems. DMT systems form their transmit signal by adding the complex conjugate of the information symbols (frequency domain) to the information symbols before taking the IDFT. This simplifies creating a real signal, but requires that at least a part of a resulting image be removed through filtering to prevent crosstalk generation.
Severe system crosstalk is caused by NEXT where upstream and downstream bands overlap inappropriately (i.e., upstream on downstream or vice versa). If NEXT is caused by the image and occurs close to a reflection point frequency around which the image is formed, filtering requirements for image removal are also severe.
Accordingly, what is needed in the art is a more effective way to reduce the effect of and preferably prevent image-generated NEXT associated with a DSL system.